For years, hearing impaired persons have struggled to obtain equipment from telephone service providers that is compatible with standard hearing aid devices. A well known result of this effort is the "tele-coil", a magnetic field enhancing device in all public telephones in the United States that allows many hearing aid amplifiers to directly couple to the output of a standard telephone handset. Until recently, devices like the tele-coil provided good compatibility for the hearing impaired with the public telephone network.
Recently, however, it has become apparent that the current generation of digital cellular telephones, if not modified, will cause tremendous problems for the hearing impaired. Cellular telephones generally operate in the 0.8 to 2.0 GHz frequency region. Analog cellular telephones have not caused any interference with hearing aids. Existing digital cellular telephones, however, use a switched carrier that cycles on and off at a frequency of 50 Hz (USDC) or 217 Hz (GSM). The switched carrier is used because USDC and GSM phones use a Time Division Multiple Access (TDMA) technique that allows 3 to 8 users to share the same frequency. The high frequency carrier has a relatively high energy when held next to a hearing aid users ear and is demodulated by the natural geometry of nearly every hearing aid on the market.
As described by Preves and by Arndt at the Hearing Aid Compatibility and Accessibility to Digital Wireless Telecommunications Summit Meeting held on Jan. 3, 1996, the internal wiring of the hearing aid acts as an antenna (or antennae) and the nonlinearity of the transistors in the amplifier stages (and in the FET preamplifier contained in the typical electret microphone) demodulates the "audio-band Morse code" modulation of the carrier. The harmonics of the 50 Hz or 217 Hz square wave modulation fall in the region of frequencies where the primary information in speech is carried, producing a masking noise. As demonstrated in a videotape prepared by the applicant and submitted to the conference, the result is often to make the cellular telephone unusable by the hearing-aid wearer.
At first, this problem was not widely noted, since digital service was not available in most areas and since all digital phones could be switched to analog, which has a continuous carrier that eliminates the audio interference. Hearing aid users who found themselves unable to communicate in the digital mode could simply switch to analog, which has a continuous carrier that eliminates the audio interference. Increasingly, however, analog service is being phased out in favor of digital service because of the larger number of users per allocated frequency that can be accommodated by digital service. Moreover, some of the newest digital phones use the European GSM version of TDMA, which does not permit analog (continuous carrier) operation but uses digital only operation where the carrier is switched at a rate of 217 times per second. Also, these new phones allow a higher level of emitted power.
At a BTE (Behind-the-ear) or ITE (full-size in-the-ear) hearing aid, the measured field strength generated by transmissions from a nearby hand-held cellular phone can be in the 3 V/m range when the hearing aid user is 1 meter away ("bystander" condition). When the cellular phone is to be used by the hearing aid wearer, however, the field strength seen by the hearing aid when the phone is brought to the ear can be in the 100-200 V/m range ("user" condition).
An informal estimate of the relative interference problems caused by the three digital phone schemes, GSM, TDMA, and Code Division Multiple Access (CDMA) is illustrated in FIG. 2. FIG. 2 shows a cartoon representation of the problem, likening the interference problem to that of driving over a road with spikes or speed bumps built in. Empirically, the worst interference is caused by the GSM system. In fact, the current driving force behind seeking a solution comes from the recent deployment of a pilot GSM system in the Washington, D.C. area.
It is possible to modify the design of such hearing aids so they become relatively immune to interference from 3 V/m "bystander" carrier strengths. Reports from engineers here and in Europe, however, indicate that it is impractical to reduce the sensitivity of many BTE and ITE hearing aid designs sufficiently to allow use of cellular phones. Many of these existing aids are intrinsically good designs from the audio and audiological standpoint, but use printed-circuit boards and hand wiring to the components and adjustment trimmers so that the minimum "antenna" size is still too large to allow adequate bypassing. Known shielding techniques, such as shielding the case with silver paint, may produce a 10-15 dB improvement. Also, it is known that bypassing each amplifier stage with a pi-filter may also provide incremental improvements. However, these approaches can only bring the effective immunity from perhaps 1-2 V/m to 10-20 V/m; still 20 dB shy of the needed immunity to the strong near field produced by digital wireless telephones held adjacent to the ear.
It is noted that In-the-Canal and Completely-in-the-Canal (CIC) aids presently in production are often completely immune to the 100-200 V/m field strengths generated by cellular telephones at the ear without special further effort. This comes about because of a combination of the shielding provided by the ear canal and the fact that their tiny size dictates use of integrated-circuit and hybrid designs where the wiring is necessarily so short that very little antenna length can exist. It is the approximately 1.5 million users of BTE hearing aids that is of primary concern of this invention, since many of these users have a hearing loss so severe that they must use BTE aids to obtain sufficient power and gain (requiring larger receivers than can fit in the canal) without feedback. Many of these users cannot hear the telephone output directly and must use their aids with the telephone.
Thus, there appears to be a need to bridge the 20-26 dB gap between the fully modulated 100-200 V/m strength of the present TDMA and GSM carriers and the perhaps 10 V/m immunity level that may be practical in BTE designs. In Europe, new hearing aids must only be immune to a 3 V/m field, enough immunity to prevent a nearby phone from interfering with a BTE hearing aid but not nearly enough immunity to allow the wearer of a BTE hearing aid to use a GSM or a United States TDMA phone.
The applicant has demonstrated empirically that CDMA does not present a significant interference problem under some conditions. However, the widespread deployment of CDMA cellular is still years away and a large installed base of TDMA cellular equipment is expected to exist for many years to come. There is a need, therefore, to make the existing cellular infrastructure compatible and accessible to the wearers of the millions of hearing aids currently in the field.